1. Field of the Invention
The present invention relates generally to sensors, and it more particularly relates to level sensors for gauging the level and condition of fluids and other materials in containers, as well as in reactors, pipes, river beds, irrigation ditches, conveyors, free standing piles, and a variety of other systems. Additionally, the present invention relates to a linear displacement transducer for use in machine tools, hydraulic actuators and similar devices with movable components.
2. Background Art
Various devices have been conventionally employed to measure the level of a liquid or the interface levels between two liquids. Generally, these devices consist of a sensor within a container, and means for sending data from the sensor to a remote location where it would be detected and converted into a usable format representative of the level of liquid within the container.
Exemplary mechanical and electromechanical sensors such as floats, are unreliable, and are difficult to use in deep tanks or tanks filled with corrosive liquids. Devices using sonic and ultrasonic signals are relatively inaccurate because the speed of sound varies with temperature and humidity. Ultrasound based sensors do not provide an accurate measurement of the true fluid level, since ultrasound signals generally reflect off foamy material (or foam) which might form on the fluid surface. In capacitive level sensors the liquid must be close to the sensor, and wall wetting can be erroneously sensed by the sensor. Also, a capacitive sensor does not provide information about the condition of the fluid or mixture in which it is immersed.
Frequency modulation (FM)-continuous wave (CW) radar sensors have also been used to measure the liquid level in a container. In these sensors, a free propagating radar signal is transmitted toward the liquid surface upon which it is reflected back to a receiver for determining the signal time of flight, and thus the distance between a reference point and the fluid level. These radar sensors are expensive, bulky, and have limited resolution, typically 6 inches.
Most conventional time domain reflectometry (TDR) systems provide accurate indications of liquid level and respond quickly to changes in the liquid levels, but they require very complex and expensive systems to function properly. One such system is described in U.S. Pat. No. 3,832,900 to Ross and utilizes an open coaxial line that is immersed in, and filled by the contained liquid. The liquid surface creates a discontinuity in the coaxial line that produces a reflection of the baseband pulse signal that propagates back along the transmission line. The time at which this reflection is received, relative to the time of the transmitted pulse, determines the liquid level. However, the transmission line tends to clog and requires frequent cleaning.
U.S. Pat. No. 3,995,212 to Ross describes an apparatus for measuring the liquid level, which aims at addressing the clogging problem and reducing the complexity of the radiation reflection detection systems. More specifically, this patent describes an apparatus formed of a pulse generator that produces a baseband probe pulse of subnanosecond duration, which is coupled to a transition device through a directional coupler. The transition device converts the transmission line guiding the probe pulse into a single wire transmission line along which the wave continues to propagate. The wire is positioned perpendicular to the surface of the liquid and extends therethrough. Reflections of the probe pulse, caused by the change in dielectric constant at the surface of the liquid, are propagated back along the wire toward the transition device and coupled to the directional coupler.
Samples of the reflected wave are coupled to the reflection port of the directional coupler which also possesses a port at which samples of the incident waves appear. These ports are coupled through a switch to a level processor in which the delay between the incident and reflected waves is determined. This delay is utilized to determine the liquid level. However, this liquid level detection system is relatively expensive and bulky, and does not enable the measurement of the depth of the electrically conductive liquids.
U.S. Pat. No. 4,489,601 to Rao et al. describes an apparatus for measuring the height of a conductive or non-conductive liquid above a reference level. A Goubau transmission line extends from an input terminal to a junction with a hollow cylinder having a diameter that is greater than the outer diameter of the Goubau line. At this junction, the Goubau line extends into the hollow cylinder to form a coaxial transmission line, which is immersed vertically in a liquid where it extends from above the surface of the liquid to some reference level below the surface, typically the bottom.
A pulse is coupled to the Goubau line at the input terminal and propagates as a surface wave until it reaches the junction where it is transformed into two modes: a surface wave mode on the outside of the coaxial line and a transverse electromagnetic mode within the coaxial line. A transition device controls the relative power distributed to the two modes. A reflection occurs when the surface wave propagating on the outside of the coaxial line encounters the surface of the liquid. The transverse electromagnetic mode remains entirely within the coaxial line and is not reflected until a later time when it reaches the reference level below the surface. The reflected surface wave on the outside of the coaxial line and the reflected transverse electromagnetic mode wave within the coaxial line propagate back along the coaxial line, and are coupled to the Goubau line as reflected surface waves separated in time. The reflected waves propagating on the Goubau line are transferred to a direction coupler and detected by a receiver. The time between the received pulses is proportional to the distance from the surface of the liquid to the reference level. However, this conventional TDR level sensing apparatus is relatively expensive and bulky.
There is therefore a great and still unsatisfied need for a new material level sensor which addresses the foregoing concerns associated with conventional devices, and which provide adequate solutions thereto. This new level sensor should be capable of gauging the level and condition of fluids and other materials in containers, as well as in reactors, pipes, river beds, irrigation ditches, conveyors, free standing piles, and a variety of other systems. Additionally, the new sensor should also be amenable for use as a linear displacement transducer in machine tools, hydraulic actuators and similar devices with movable components.